A Study  of  Soap  Solutions. 

IT 


By  H.  W.  Hillyer. 


Y\S^* 


[ Reprinted  from  the  journal  of  the  American  Chemical  Society, 
Vol.  XXV,  No.  5.  May,  1903.I 


A STUDY  OF  SOAP  SOLUTIONS. 

By  H.  W.  Hillyer. 

Received  March  20,  1903. 

It  was  noted  in  the  preceding  article  that  while  toward  air  the 
surface-tensions  of  soap  solutions  vary  but  little  as  the  concentra- 
tion is  changed,  the  surface-tension  between  soap  solutions  and  oils 
is  rapidly  diminished  as  the  concentration  is  increased.  To 
ascertain  the  law  connecting  concentration  and  surface-tension, 
the  following  experiments  were  made.  They  not  only  show  this 
relation  but  serve  to  confirm  the  idea  that  the  diminution  of  sur- 
face-tension is  dependent  solely  on  the  amount  of  undecomposed 
soap  present  in  the  solution.  Unfortunately  in  this  pioneer  work 
it  was  not  realized  how  desirable  it  was  to  have  the  same  kerosene 
for  all  experiments,  and  consequently  the  results  are  not  strictly 


My OD  t.SS 


A STUDY  OF  SOAP  SOLUTIONS. 


525 


and  numerically  comparable  except  in  the  cases  which  are  plotted 
together.  But  it  is  thought  best  at  this  time  to  publish  these  re- 
sults, as  their  general  tendency  is  quite  clear  and  as  they  serve  to 
indicate  the  basis  on  which  rests  the  method  for  evaluating  soaps 
which  will  soon  be  published. 

A decinormal  solution  of  sodium  oleate  was  prepared  by  dis- 
solving 14. 1 grams  of  oleic  acid  in  500  cc.  decinormal  sodium 
hydroxide  solution.  The  solution  was  almost  completely  clear. 
It  was  then  filtered.  The  solution  was  diluted  successively  to 
lower  concentrations  and  the  number  of  drops  formed  when  these 
various  solutions  were  passed  through  kerosene  was  noted. 
Duplicate  counts  were  made  and  the  number  of  drops  found  in  the 
separate  counts  are  indicated. 


Sodium  Oleate. 


Concentration. 

N/10. 

N/20. 

N/40. 

N/80.  N/160.  N/320. 

N/640.  Water. 

No.  drops 

f 464 
1460 

269 

270 

184 

l8l 

Il6 

Il6 

80 

78 

5i 

54 

35  20 

Another  set  of  solutions-  - 

292 

184 

Il8 

124 

74 

77 

49 

47 

32 

3i 

In  the  diagram  (Fig.  1,  a ) the  ordinates  show  the  number  of 
drops  formed  and  the  abscissas  show  the  concentrations.  The 
number  of  drops  is  not  proportional  to  the  concentration  but  falls 


P* 


19331 


526 


H.  W.  HILLYER. 


below  proportionality  as  the  concentration  increases.  In  another 
experiment,  the  solutions  were  very  carefully  made  and  after 
preparation  were  placed  in  stoppered  flasks  in  a large  tank  of 
water  that  they  might  be  kept  as  nearly  as  possible  at  an  average 
room  temperature.  The  special  object  of  this  experiment  was  to 
ascertain  whether  any  change  took  place  on  standing  which  would 
indicate  that  a hydrolysis  of  the  soap  was  gradually  going  on.  In 
this  case  the  kerosene  was  the  same  throughout  the  experiment, 
but  different  from  that  used  in  the  last.  For  each  count  the  kero- 
sene was  changed  except  where  the  solutions  were  very  dilute. 
The  solutions  of  various  concentration  were  prepared  as  rapidly  as 
possible  and  then  the  counts  made.  After  this  the  solutions  were 
kept  at  the  temperature  of  the  tank  of  water  and  the  counts  re- 
peated from  day  to  day. 


Sodium  Oeeate,  Time  and  Dilution. 


Concentration.  N/io.  N/20. 

December  2nd 297  196 

December  3rd 296  200 

December  5th 194 

December  8th 301 


N/40. 

N/80. 

N/160. 

N/320. 

N/640. 

Water. 

125 

88 

63 

4i 

26i 

14 

126 

87 

61 

40 1 

261 

14 

123 

85 

62 

40? 

261 

•• 

132 

59s 

261 

From  this  it  seems  that  in  the  course  of  six  days  no  change  takes 
place  in  the  surface-tension.  There  is  no  evidence  of  any  decompo- 
sition of  the  soap  by  a diminution  of  the  number  of  drops  in 
either  strong  or  dilute  solutions.  The  curve  (Fig.  I,  b)  showing 
the  relation  of  concentration  to  number  of  drops  is  here  shown  in 
connection  with  that  given  above  that  it  may  be  seen  that  the  two 
have  the  same  shape.  They  should  not,  however,  be  compared 
numerically  except  to  enforce  the  fact  that  the  effects  are  quite 
different  with  different  samples  of  kerosene.  When  sodium  oleate 
solutions  stand  in  uncorked  flasks,  the  carbon  dioxide  of  the  air 
decomposes  the  soap  and  the  number  of  drops  formed  by  a given 
solution  gradually  diminishes.  This  is  especially  marked  in  case 
of  dilute  solutions. 

SODIUM  PAEMITATE  AND  STEARATE. 

Pure  sodium  palmitate  and  stearate  are  very  little  soluble  in 
cold  water,  so  little  in  fact  that  it  is  difficult  by  shaking  them  with 


A STUDY  OF  SOAP  SOLUTIONS. 


527 


cold  water  to  produce  a foam.  Palmitate  and  stearate,  when 
rubbed  on  the  hands  with  cold  distilled  water,  do  not  give  the  effect 
of  soap  but  seem  more  nearly  like  paraffin  or  tallow.  On  account 
of  this  small  solubility  in  cold  water,  it  was  necessary  to  devise  a 
method  of  working  at  a higher  temperature.  The  apparatus 
finally  used  is  very  simple.  It  consists  of  a beaker  of  about  700 
cc.  capacity  provided  with  a cover  of  zinc  with  its  edges  turned 
down.  In  the  cover  are  three  holes  one  for  the  thermometer  and 
two  large  enough  to  receive  short  wide  test-tubes.  One  of  the 
test-tubes  serves  as  a reservoir  to  hold  the  hot  soap  solution 
to  be  used,  while  the  other  is  the  working  part  of  the  apparatus. 
After  the  water  in  the  beaker  has  been  heated  to  boiling,  a supply 
of  the  solution  to  be  tested,  which  has  been  heated  to  bring  the 
soap  into  solution,  is  placed  in  the  reservoir  tube.  Then  15  cc. 
of  kerosene  are  placed  in  the  working  tube  and  the  pipette  put  in. 
When  all  is  thoroughly  heated  to  the  boiling  temperature,  the 
pipette  is  filled  with  the  solution,  placed  with  its  end  below  the  oil 
and  the  number  of  drops  counted.  At  first,  great  difficulty  was 


Fig  2. 


encountered  in  getting  concordant  results  but  it  was  learned  that 
when  the  lower  end  of  the  pipette  was  ground  off  the  numerous 
minor  capillary  tubes  in  the  glass  of  the  lower  part  of  the  pipette 
continually  gave  out  small  amounts  of  air  and  when  this  formed 
into  bubbles  on  the  end  of  the  pipette  it  caused  an  increase  in  the 
number  and  decrease  in  the  size  of  the  drops  which  were  forming. 
As  this  exudation  of  air  was  irregular  and  introduced  a new  factor 
in  the  surface-tension  problem  a means  was  necessary  to  prevent  it. 
The  remedy  was  to  so  shape  the  pipette  that  the  lower  end  after 
grinding  could  be  heated  to  the  point  of  fusion  until  all  the  small 
capillaries  were  closed  without  closing  the  main  capillary  or  nar- 
rowing too  much  the  surface  on  which  the  drops  were  to  form. 
The  form  finally  worked  out  is  shown  in  the  accompanying 
sketch  (Fig.  2). 


528 


H.  W.  HILLYER. 


Twentieth-normal  sodium  palmitate  was  made  by  treating  12.8 
grams  palmitic  acid  with  500  cc.  decinormal  sodium  hydroxide 
and  500  cc.  water,  and  heating  all  on  the  water-bath.  The  solu- 
tion was  quite  clear.  Small  quantities  of  palmitic  acid  were  added 
until  oil  droplets  were  plainly  visible  and  the  solution  was 
filtered.  This  solution  was  then  diluted  with  hot  water  to  make 
the  concentrations  used  below  and  these  were  kept  on  the  steam- 
bath  at  6o°-70°.  None  of  them  were  more  than  slightly  cloudy 
at  6o°-70°.  They  were  tested  in  the  apparatus  just  described,  and 
gave  the  number  of  drops  here  set  down. 


Sodium  Paumitate. 


Concentration.  N/20.  N/40.  N/80.  N/160.  N/320.  N/640.  N/1280. 


No-  drops {*♦  3*5  94 

On  plotting  these  data,  the  curve  (Fig.  3,  a) 


49  32  23 
49  33  23^ 

results.  The  sag 


in  the  curve  near  its  origin  was  thought  so  remarkable  that  a 
second  set  of  solutions  was  prepared  and  tested. 

N/20.  N/30.  N/40.  n/8o.  N/160.  N/320.  N/640.  N/12P0. 

403  368  308  208  90  40  31  24 

When  these  data  are  plotted,  a curve  is  obtained  in  shape  like 
that  obtained  before  and  practically  indentical  with  it.  If  the 
same  law  connected  concentration  with  the  number  of  drops  at 
low  as  at  high  concentration,  the  curve  would  be  a smooth  one 
extending  to  the  point  showing  the  number  of  drops  made  by 
water,  as  indicated  by  the  dotted  line.  That  the  curve  falls  below 


A study  of  soap  solutions. 


529 


this  line  may  be  explained  by  supposing  that  in  the  dilute  solutions 
a marked  hydrolysis  takes  place.  Krafft  and  Stern  found  that  it 
was  necessary  to  dilute  a solution  of  sodium  palmitate  till  it  con- 
tained only  about  1 part  in  1000  before  the  precipitate  formed  on 
cooling  had  the  composition  of  the  true  acid  salt,  C16H3102Na, 
C16H3202.  It  is  to  be  noted  that  the  greatest  depression  in  the 
curve  above  is  in  the  region  where  the  concentration  is  near  to  1 
part  in  1000  of  water.  This  point  is  marked  by  the  arrow.  If 
this  is  a true  explanation,  it  is  an  interesting  confirmation  of  the 
supposition  that  the  number  of  drops  is  a function  of  the  amount 
of  undecomposed  soap  present.  The  hydrolysis  lessens  the 
amount  of  soap,  and  farther,  the  acid  salt  may  in  this  case,  as  in 
the  case  of  sodium  oleate,  actually  diminish  the  number  of  drops 
which  would  be  formed  if  the  soap  were  alone.  It  would  seem 
that  we  have  here  a proof  that  actual  hydrolysis  takes  place  while 
all  of  the  substances  are  in  solution.  At  the  temperature  at 
which  the  work  was  done  the  solutions  are  quite  clear  and  show 
no  sign  of  a precipitate. 

SODIUM  STEARATE. 

Sodium  stearate  solution  (one-twentieth  normal)  was  made  by 
dissolving  2.84  grams  chemically  pure  stearic  acid  in  100  cc.  hot 
decinormal  sodium  hydroxide  solution  and  diluting  to  200  cc. 
with  hot  water.  The  dilute  solutions  were  made  as  before  and 
the  number  of  drops  made  by  each  determined  at  ioo°.  None  of 
the  solutions  at  ioo°  show  more  than  a very  slight  opalescence. 
The  number  of  drops  formed  are  shown  in  the  table : 

Sodium  Stearate. 


N/ao. 

N/40. 

N/80. 

N/160. 

N/320. 

N/640. 

N/1280. 

328 

213 

H3 

} 72  { 

38 

31 

29 

340 

228 

149 

40 

28 

33 

The  curve  (Fig.  6,  b)  showing  these  numbers  in  a graphic  way 
has  very  much  the  same  shape  as  that  for  sodium  palmitate  and 
here  also  the  greatest  hydrolysis  is  apparently  at  a point  where 
there  is  present  about  1 part  of  the  salt  to  1000  of  water.  By 
comparing  the  shape  of  these  curves  with  that  of  sodium  oleate 
at  ioo°  it  becomes  evident  that  their  peculiar  form  is  not  due  to 
the  action  of  the  higher  temperature.  As  a passing  observation 
it  may  be  noted  that  when  the  solutions  stand  in  the  cold,  the  pre- 
cipitates formed  in  the  N/160  solution  and  those  of  greater  con- 


530 


H.  W.  HILLYER. 


centration,  float  in  the  solutions,  while  the  precipitates  formed  in 
the  N/320  solution  and  those  of  less  concentration  sink  in  the 
solutions,  this  change  of  relative  density  also  taking  place  when 
the  solution  contains  about  1 part  of  sodium  stearate  in  1000  of 
water. 

SODIUM  OEEATE  AT  100°. 

Sodium  oleate  solutions  when  tested  at  ioo°  show  no  special 
change  in  the  form  of  the  curve  connecting  concentration  and 
number  of  drops  from  that  belonging  to  the  oleate  in  the  cold. 
Since  the  data  were  obtained  using  the  same  apparatus  and  same 
kerosene  as  that  used  for  the  palmitate  and  stearate  they  are 
plotted  in  the  same  diagram  at  c (Fig.  3). 

ROSIN  SOAP. 

The  soap  made  by  action  of  alkalies  on  colophony  is  so  common 
a constituent  of  yellow  soaps  that  a study  of  its  solutions  is  of 
some  interest,  especially  if  we  may  gain  knowledge  of  the  reason 
of  the  disfavor  with  which  the  soap  is  regarded.  A solution  of 
rosin  soap  approximately  decinormal  was  prepared  by  digesting 
in  the  cold  powdered  rosin  in  excess  with  decinormal  sodium 
hydroxide  solution  as  long  as  any  rosin  was  dissolved.  This 
solution  was  diluted  with  freshly  boiled  cold  water  to  give  the 
concentrations  indicated  below  and  each  tested  in  the  cold  to 
learn  the  number  of  drops  made  by  each. 


Rosin  Soap  in  the  Coed. 


Concentration. 

N/10. 

N/20. 

N/30. 

N/40. 

N/60. 

N/80.  N/160. 

N/320.  N/640. 

No.  drops 

/ 361 

236 

201 

*5T 

118 

78 

47 

32 

26 

1353 

243 

207 

155 

123 

79 

46 

34 

25 

These  results  are  plotted  in  curve  a (Fig.  4).  The  same  solu- 
tions were  tested  at  ioo°,  using  the  same  pipette  and  same  kero- 
sene. 


Rosin  Soap,  at  ioo°. 


Concentration. 

N/10. 

N/20. 

N/30. 

N/40. 

N/60. 

N/80. 

N/320. 

N/640. 

No.  drops 

. I 288 

215 

183 

123 

68 

45 

35 

29 

\ 289 

215 

187 

127 

67 

45 

35 

28 

These  results  are  plotted  in  curve  b (Fig.  4).  The  notable 
thing  in  both  curves  is  the  sag  near  the  origin.  The  more  dilute 
solutions  give  a smaller  number  of  drops  than  might  be  expected 
if  the  same  law  held  that  governs  with  higher  concentrations.  If, 
as  is  indicated  by  the  facts  given  in  this  and  the  preceding  paper, 
the  number  of  drops  is  a measure  of  the  amount  of  undecomposed 


A STUDY  OF  SOAP  SOLUTIONS. 


531 


soap  present  in  solution,  it  seems  that  with  the  rosin  soap  as  with 
the  palmitate  and  stearate  a marked  hydrolysis  takes  place  in  dilute 
solutions.  The  solutions  with  the  exception  of  the  decinormal 


and  one-twentieth  normal  are  much  clouded,  even  in  the  cold, 
showing  by  this  also  that  hydrolysis  has  taken  place.  The  hot 
solutions,  even  the  most  concentrated,  are  cloudy.  The  hydrolysis 
indicated  by  this  cloudiness  is  also  shown  by  the  fact  that  the 
curve  for  the  hot  solutions  lies  below  that  for  the  cold  solutions. 
Elevation  of  temperature  is  favorable  to  diminution  of  surface- 
tension  and  a greater  number  of  drops  might  be  expected  with  all 
these  solutions  at  the  higher  temperature  but,  as  a smaller  number 
is  found,  the  explanation  lies  in  the  hydrolysis  caused  by  heating 
the  solutions. 

In  looking  over  the  results  obtained,  it  is  of  interest  to  note  their 
agreement  with  well-known  facts  in  regard  to  the  soaps.  The 
oleate  is  very  soluble  in  cold  water  and  even  in  dilute  solutions 
shows  little  hydrolysis.  Soaps  rich  in  oleate  are  useful  for  toilet 
purposes  and  for  wool-scouring  when  cold  water  is  used.  Its 
stability  in  dilute  solutions  would  make  it  of  value  as  a detergent 
even  until  it  is  completely  rinsed  away  and  its  ready  solubility 
would  make  it  easy  to  wash  away. 

For  laundry  work,  dish-washing,  and  other  work,  when  hot  water 
is  used,  practical  experience  shows  that  the  most  desirable  soap  is  a 
tallow  soap.  This  is  rich  in  palmitate  and  stearate  with  some  oleate. 
As  shown  by  the  curves,  the  palmitate  and  stearate  have  a very 
high  efficiency  at  a high  temperature,  especially  when  the  concen- 


532 


H.  W.  HILLYER. 


tration  is  great.  When  the  concentration  is  lessened,  marked 
hydrolysis  takes  place  and  the  efficiency  rapidly  falls  off,  but  with 
mixed  soaps  this  low  efficiency  of  the  palmitate  and  stearate  is 
supplemented  by  the  relatively  high  efficiency  of  the  oleate  in 
dilute  solutions,  which  will  sustain  the  detergent  effect  until  all 
impurities  are  washed  away,  including  the  acid  palmitate  and 
stearate  which  might  otherwise  be  retained  by  the  fabric.  When 
the  temperature  is  low,  palmitate  and  stearate  are  so  little  soluble 
as.  to  be  of  no  practical  value,  since  the  only  effect  of  water  on 
them  is  to  hydrolyze  them  and  set  free  a small  quantity  of  alkali 
which,  according  to  the  hypothesis  here  favored,  has  no  detergent 
effect  on  neutral  oils. 

Rosin  soap  is  usually  regarded  as  a comparatively  undesirable 
ingredient  of  soaps.  By  a test  with  decinormal  solution,  it  is  of 
about  the  same  efficiency  according  to  the  dropping  test  in  kerosene 
as  sodium  oleate.  But  when  the  dilution  curves  are  studied,  it  is 
seen  that  dilute  solutions,  especially  in  the  heat,  show  marked 
hydrolysis  which  is  necessarily  accompanied  by  separation  of  the 
rosin  acids.  Here  the  acid  product  of  hydrolysis  separates  in  a 
cloud  and  does  not  stay  in  solution  as  with  the  palmitate  and 
stearate  in  the  heat.  The  separated  rosin  acids  may  well  settle 
on  the  fabric  being  washed  and  impart  to  it  the  odor  of  rosin, 
cause  it  to  be  yellow  and  make  it  ready  to  easily  take  up  dust. 
This  effect  may  be  partially  offset  in  using  mixed  soaps,  by  the 
other  ingredients  whose  detergent  action  will  tend  to  remove  the 
rosin  acids,  but  the  evil  effect  will  still,  to  some  extent,  remain. 

In  this  work,  when  kerosene  is  used  as  the  test  substance,  it  is, 
of  course,  true  that  we  are  really  measuring  the  detergent  effect 
of  the  various  soaps  toward  kerosene  only.  Some  few  observa- 
tions show  that  toward  other  fats  and  oils  the  relative  efficiency 
of  the  different  soaps  is  different.  It  may  be  necessary  to  take 
this  into  account  when  judging  the  value  of  a soap  for  some 
special  purpose. 

LABORATORY  OF  ORGANIC  CHEMISTRY,  UNIVERSITY 

of  Wisconsin,  March  17,  1902. 


